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Chapter II
HABITS OF WORMS--continued.
Manner in which worms seize objects--Their power of suction--The
instinct of plugging up the mouths of their burrows--Stones piled
over the burrows--The advantages thus gained--Intelligence shown by
worms in their manner of plugging up their burrows--Various kinds
of leaves and other objects thus used--Triangles of paper--Summary
of reasons for believing that worms exhibit some intelligence--
Means by which they excavate their burrows, by pushing away the
earth and swallowing it--Earth also swallowed for the nutritious
matter which it contains--Depth to which worms burrow, and the
construction of their burrows--Burrows lined with castings, and in
the upper part with leaves--The lowest part paved with little
stones or seeds--Manner in which the castings are ejected--The
collapse of old burrows--Distribution of worms--Tower-like castings
in Bengal--Gigantic castings on the Nilgiri Mountains--Castings
ejected in all countries.
In the pots in which worms were kept, leaves were pinned down to
the soil, and at night the manner in which they were seized could
be observed. The worms always endeavoured to drag the leaves
towards their burrows; and they tore or sucked off small fragments,
whenever the leaves were sufficiently tender. They generally
seized the thin edge of a leaf with their mouths, between the
projecting upper and lower lip; the thick and strong pharynx being
at the same time, as Perrier remarks, pushed forward within their
bodies, so as to afford a point of resistance for the upper lip.
In the case of broad flat objects they acted in a wholly different
manner. The pointed anterior extremity of the body, after being
brought into contact with an object of this kind, was drawn within
the adjoining rings, so that it appeared truncated and became as
thick as the rest of the body. This part could then be seen to
swell a little; and this, I believe, is due to the pharynx being
pushed a little forwards. Then by a slight withdrawal of the
pharynx or by its expansion, a vacuum was produced beneath the
truncated slimy end of the body whilst in contact with the object;
and by this means the two adhered firmly together. {28} That under
these circumstances a vacuum was produced was plainly seen on one
occasion, when a large worm lying beneath a flaccid cabbage leaf
tried to drag it away; for the surface of the leaf directly over
the end of the worm's body became deeply pitted. On another
occasion a worm suddenly lost its hold on a flat leaf; and the
anterior end of the body was momentarily seen to be cup-formed.
Worms can attach themselves to an object beneath water in the same
manner; and I saw one thus dragging away a submerged slice of an
onion-bulb.
The edges of fresh or nearly fresh leaves affixed to the ground
were often nibbled by the worms; and sometimes the epidermis and
all the parenchyma on one side was gnawed completely away over a
considerable space; the epidermis alone on the opposite side being
left quite clean. The veins were never touched, and leaves were
thus sometimes partly converted into skeletons. As worms have no
teeth and as their mouths consist of very soft tissue, it may be
presumed that they consume by means of suction the edges and the
parenchyma of fresh leaves, after they have been softened by the
digestive fluid. They cannot attack such strong leaves as those of
sea-kale or large and thick leaves of ivy; though one of the latter
after it had become rotten was reduced in parts to the state of a
skeleton.
Worms seize leaves and other objects, not only to serve as food,
but for plugging up the mouths of their burrows; and this is one of
their strongest instincts. They sometimes work so energetically
that Mr. D. F. Simpson, who has a small walled garden where worms
abound in Bayswater, informs me that on a calm damp evening he
there heard so extraordinary a rustling noise from under a tree
from which many leaves had fallen, that he went out with a light
and discovered that the noise was caused by many worms dragging the
dry leaves and squeezing them into the burrows. Not only leaves,
but petioles of many kinds, some flower-peduncles, often decayed
twigs of trees, bits of paper, feathers, tufts of wool and horse-
hairs are dragged into their burrows for this purpose. I have seen
as many as seventeen petioles of a Clematis projecting from the
mouth of one burrow, and ten from the mouth of another. Some of
these objects, such as the petioles just named, feathers, &c., are
never gnawed by worms. In a gravel-walk in my garden I found many
hundred leaves of a pine-tree (P. austriaca or nigricans) drawn by
their bases into burrows. The surfaces by which these leaves are
articulated to the branches are shaped in as peculiar a manner as
is the joint between the leg-bones of a quadruped; and if these
surfaces had been in the least gnawed, the fact would have been
immediately visible, but there was no trace of gnawing. Of
ordinary dicotyledonous leaves, all those which are dragged into
burrows are not gnawed. I have seen as many as nine leaves of the
lime-tree drawn into the same burrow, and not nearly all of them
had been gnawed; but such leaves may serve as a store for future
consumption. Where fallen leaves are abundant, many more are
sometimes collected over the mouth of a burrow than can be used, so
that a small pile of unused leaves is left like a roof over those
which have been partly dragged in.
A leaf in being dragged a little way into a cylindrical burrow is
necessarily much folded or crumpled. When another leaf is drawn
in, this is done exteriorly to the first one, and so on with the
succeeding leaves; and finally all become closely folded and
pressed together. Sometimes the worm enlarges the mouth of its
burrow, or makes a fresh one close by, so as to draw in a still
larger number of leaves. They often or generally fill up the
interstices between the drawn-in leaves with moist viscid earth
ejected from their bodies; and thus the mouths of the burrows are
securely plugged. Hundreds of such plugged burrows may be seen in
many places, especially during the autumnal and early winter
months. But, as will hereafter be shown, leaves are dragged into
the burrows not only for plugging them up and for food, but for the
sake of lining the upper part or mouth.
When worms cannot obtain leaves, petioles, sticks, &c., with which
to plug up the mouths of their burrows, they often protect them by
little heaps of stones; and such heaps of smooth rounded pebbles
may frequently be seen on gravel-walks. Here there can be no
question about food. A lady, who was interested in the habits of
worms, removed the little heaps of stones from the mouths of
several burrows and cleared the surface of the ground for some
inches all round. She went out on the following night with a
lantern, and saw the worms with their tails fixed in their burrows,
dragging the stones inwards by the aid of their mouths, no doubt by
suction. "After two nights some of the holes had 8 or 9 small
stones over them; after four nights one had about 30, and another
34 stones." {29} One stone--which had been dragged over the
gravel-walk to the mouth of a burrow weighed two ounces; and this
proves how strong worms are. But they show greater strength in
sometimes displacing stones in a well-trodden gravel-walk; that
they do so, may be inferred from the cavities left by the displaced
stones being exactly filled by those lying over the mouths of
adjoining burrows, as I have myself observed.
Work of this kind is usually performed during the night; but I have
occasionally known objects to be drawn into the burrows during the
day. What advantage the worms derive from plugging up the mouths
of their burrows with leaves, &c., or from piling stones over them,
is doubtful. They do not act in this manner at the times when they
eject much earth from their burrows; for their castings then serve
to cover the mouths. When gardeners wish to kill worms on a lawn,
it is necessary first to brush or rake away the castings from the
surface, in order that the lime-water may enter the burrows. {30}
It might be inferred from this fact that the mouths are plugged up
with leaves, &c., to prevent the entrance of water during heavy
rain; but it may be urged against this view that a few, loose,
well-rounded stones are ill-adapted to keep out water. I have
moreover seen many burrows in the perpendicularly cut turf-edgings
to gravel-walks, into which water could hardly flow, as well
plugged as burrows on a level surface. It is not probable that the
plugs or piles of stones serve to conceal the burrows from
scolopendras, which, according to Hoffmeister, {31} are the
bitterest enemies of worms, or from the larger species of Carabus
and Staphylinus which attack them ferociously, for these animals
are nocturnal, and the burrows are opened at night. May not worms
when the mouth of the burrow is protected be able to remain with
safety with their heads close to it, which we know that they like
to do, but which costs so many of them their lives? Or may not the
plugs check the free ingress of the lowest stratum of air, when
chilled by radiation at night, from the surrounding ground and
herbage? I am inclined to believe in this latter view: firstly,
because when worms were kept in pots in a room with a fire, in
which case cold air could not enter the burrows, they plugged them
up in a slovenly manner; and secondarily, because they often coat
the upper part of their burrows with leaves, apparently to prevent
their bodies from coming into close contact with the cold damp
earth. Mr. E. Parfitt has suggested to me that the mouths of the
burrows are closed in order that the air within them may be kept
thoroughly damp, and this seems the most probable explanation of
the habit. But the plugging-up process may serve for all the above
purposes.
Whatever the motive may be, it appears that worms much dislike
leaving the mouths of their burrows open. Nevertheless they will
reopen them at night, whether or not they can afterwards close
them. Numerous open burrows may be seen on recently-dug ground,
for in this case the worms eject their castings in cavities left in
the ground, or in the old burrows instead of piling them over the
mouths of their burrows, and they cannot collect objects on the
surface by which the mouths might be protected. So again on a
recently disinterred pavement of a Roman villa at Abinger
(hereafter to be described) the worms pertinaciously opened their
burrows almost every night, when these had been closed by being
trampled on, although they were rarely able to find a few minute
stones wherewith to protect them.
Intelligence shown by worms in their manner of plugging up their
burrows.--If a man had to plug up a small cylindrical hole, with
such objects as leaves, petioles or twigs, he would drag or push
them in by their pointed ends; but if these objects were very thin
relatively to the size of the hole, he would probably insert some
by their thicker or broader ends. The guide in his case would be
intelligence. It seemed therefore worth while to observe carefully
how worms dragged leaves into their burrows; whether by their tips
or bases or middle parts. It seemed more especially desirable to
do this in the case of plants not natives to our country; for
although the habit of dragging leaves into their burrows is
undoubtedly instinctive with worms, yet instinct could not tell
them how to act in the case of leaves about which their progenitors
knew nothing. If, moreover, worms acted solely through instinct or
an unvarying inherited impulse, they would draw all kinds of leaves
into their burrows in the same manner. If they have no such
definite instinct, we might expect that chance would determine
whether the tip, base or middle was seized. If both these
alternatives are excluded, intelligence alone is left; unless the
worm in each case first tries many different methods, and follows
that alone which proves possible or the most easy; but to act in
this manner and to try different methods makes a near approach to
intelligence.
In the first place 227 withered leaves of various kinds, mostly of
English plants, were pulled out of worm-burrows in several places.
Of these, 181 had been drawn into the burrows by or near their
tips, so that the foot-stalk projected nearly upright from the
mouth of the burrow; 20 had been drawn in by their bases, and in
this case the tips projected from the burrows; and 26 had been
seized near the middle, so that these had been drawn in
transversely and were much crumpled. Therefore 80 per cent.
(always using the nearest whole number) had been drawn in by the
tip, 9 per cent. by the base or foot-stalk, and 11 per cent.
transversely or by the middle. This alone is almost sufficient to
show that chance does not determine the manner in which leaves are
dragged into the burrows.
Of the above 227 leaves, 70 consisted of the fallen leaves of the
common lime-tree, which is almost certainly not a native of
England. These leaves are much acuminated towards the tip, and are
very broad at the base with a well-developed foot-stalk. They are
thin and quite flexible when half-withered. Of the 70, 79 per
cent. had been drawn in by or near the tip; 4 per cent. by or near
the base; and 17 per cent. transversely or by the middle. These
proportions agree very closely, as far as the tip is concerned,
with those before given. But the percentage drawn in by the base
is smaller, which may be attributed to the breadth of the basal
part of the blade. We here, also, see that the presence of a foot-
stalk, which it might have been expected would have tempted the
worms as a convenient handle, has little or no influence in
determining the manner in which lime leaves are dragged into the
burrows. The considerable proportion, viz., 17 per cent., drawn in
more or less transversely depends no doubt on the flexibility of
these half-decayed leaves. The fact of so many having been drawn
in by the middle, and of some few having been drawn in by the base,
renders it improbable that the worms first tried to draw in most of
the leaves by one or both of these methods, and that they
afterwards drew in 79 per cent. by their tips; for it is clear that
they would not have failed in drawing them in by the base or
middle.
The leaves of a foreign plant were next searched for, the blades of
which were not more pointed towards the apex than towards the base.
This proved to be the case with those of a laburnum (a hybrid
between Cytisus alpinus and laburnum) for on doubling the terminal
over the basal half, they generally fitted exactly; and when there
was any difference, the basal half was a little the narrower. It
might, therefore, have been expected that an almost equal number of
these leaves would have been drawn in by the tip and base, or a
slight excess in favour of the latter. But of 73 leaves (not
included in the first lot of 227) pulled out of worm-burrows, 63
per cent. had been drawn in by the tip; 27 per cent. by the base,
and 10 per cent. transversely. We here see that a far larger
proportion, viz., 27 per cent. were drawn in by the base than in
the case of lime leaves, the blades of which are very broad at the
base, and of which only 4 per cent. had thus been drawn in. We may
perhaps account for the fact of a still larger proportion of the
laburnum leaves not having been drawn in by the base, by worms
having acquired the habit of generally drawing in leaves by their
tips and thus avoiding the foot-stalk. For the basal margin of the
blade in many kinds of leaves forms a large angle with the foot-
stalk; and if such a leaf were drawn in by the foot-stalk, the
basal margin would come abruptly into contact with the ground on
each side of the burrow, and would render the drawing in of the
leaf very difficult.
Nevertheless worms break through their habit of avoiding the foot-
stalk, if this part offers them the most convenient means for
drawing leaves into their burrows. The leaves of the endless
hybridised varieties of the Rhododendron vary much in shape; some
are narrowest towards the base and others towards the apex. After
they have fallen off, the blade on each side of the midrib often
becomes curled up while drying, sometimes along the whole length,
sometimes chiefly at the base, sometimes towards the apex. Out of
28 fallen leaves on one bed of peat in my garden, no less than 23
were narrower in the basal quarter than in the terminal quarter of
their length; and this narrowness was chiefly due to the curling in
of the margins. Out of 36 fallen leaves on another bed, in which
different varieties of the Rhododendron grew, only 17 were narrower
towards the base than towards the apex. My son William, who first
called my attention to this case, picked up 237 fallen leaves in
his garden (where the Rhododendron grows in the natural soil) and
of these 65 per cent. could have been drawn by worms into their
burrows more easily by the base or foot-stalk than by the tip; and
this was partly due to the shape of the leaf and in a less degree
to the curling in of the margins: 27 per cent. could have been
drawn in more easily by the tip than by the base: and 8 per cent.
with about equal ease by either end. The shape of a fallen leaf
ought to be judged of before one end has been drawn into a burrow,
for after this has happened, the free end, whether it be the base
or apex, will dry more quickly than the end imbedded in the damp
ground; and the exposed margins of the free end will consequently
tend to become more curled inwards than they were when the leaf was
first seized by the worm. My son found 91 leaves which had been
dragged by worms into their burrows, though not to a great depth;
of these 66 per cent. had been drawn in by the base or foot-stalk;
and 34 per cent, by the tip. In this case, therefore, the worms
judged with a considerable degree of correctness how best to draw
the withered leaves of this foreign plant into their burrows;
notwithstanding that they had to depart from their usual habit of
avoiding the foot-stalk.
On the gravel-walks in my garden a very large number of leaves of
three species of Pinus (P. austriaca, nigricans and sylvestris) are
regularly drawn into the mouths of worm burrows. These leaves
consist of two so-called needles, which are of considerable length
in the two first and short in the last named species, and are
united to a common base; and it is by this part that they are
almost invariably drawn into the burrows. I have seen only two or
at most three exceptions to this rule with worms in a state of
nature. As the sharply pointed needles diverge a little, and as
several leaves are drawn into the same burrow, each tuft forms a
perfect chevaux de frise. On two occasions many of these tufts
were pulled up in the evening, but by the following morning fresh
leaves had been pulled in, and the burrows were again well
protected. These leaves could not be dragged into the burrows to
any depth, except by their bases, as a worm cannot seize hold of
the two needles at the same time, and if one alone were seized by
the apex, the other would be pressed against the ground and would
resist the entry of the seized one. This was manifest in the above
mentioned two or three exceptional cases. In order, therefore,
that worms should do their work well, they must drag pine-leaves
into their burrows by their bases, where the two needles are
conjoined. But how they are guided in this work is a perplexing
question.
This difficulty led my son Francis and myself to observe worms in
confinement during several nights by the aid of a dim light, while
they dragged the leaves of the above named pines into their
burrows. They moved the anterior extremities of their bodies about
the leaves, and on several occasions when they touched the sharp
end of a needle they withdrew suddenly as if pricked. But I doubt
whether they were hurt, for they are indifferent to very sharp
objects, and will swallow even rose-thorns and small splinters of
glass. It may also be doubted, whether the sharp ends of the
needles serve to tell them that this is the wrong end to seize; for
the points were cut off many leaves for a length of about one inch,
and fifty-seven of them thus treated were drawn into the burrows by
their bases, and not one by the cut-off ends. The worms in
confinement often seized the needles near the middle and drew them
towards the mouths of their burrows; and one worm tried in a
senseless manner to drag them into the burrow by bending them.
They sometimes collected many more leaves over the mouths of their
burrows (as in the case formerly mentioned of lime-leaves) than
could enter them. On other occasions, however, they behaved very
differently; for as soon as they touched the base of a pine-leaf,
this was seized, being sometimes completely engulfed in their
mouths, or a point very near the base was seized, and the leaf was
then quickly dragged or rather jerked into their burrows. It
appeared both to my son and myself as if the worms instantly
perceived as soon as they had seized a leaf in the proper manner.
Nine such cases were observed, but in one of them the worm failed
to drag the leaf into its burrow, as it was entangled by other
leaves lying near. In another case a leaf stood nearly upright
with the points of the needles partly inserted into a burrow, but
how placed there was not seen; and then the worm reared itself up
and seized the base, which was dragged into the mouth of the burrow
by bowing the whole leaf. On the other hand, after a worm had
seized the base of a leaf, this was on two occasions relinquished
from some unknown motive.
As already remarked, the habit of plugging up the mouths of the
burrows with various objects, is no doubt instinctive in worms; and
a very young one, born in one of my pots, dragged for some little
distance a Scotch-fir leaf, one needle of which was as long and
almost as thick as its own body. No species of pine is endemic in
this part of England, it is therefore incredible that the proper
manner of dragging pine-leaves into the burrows can be instinctive
with our worms. But as the worms on which the above observations
were made, were dug up beneath or near some pines, which had been
planted there about forty years, it was desirable to prove that
their actions were not instinctive. Accordingly, pine-leaves were
scattered on the ground in places far removed from any pine-tree,
and 90 of them were drawn into the burrows by their bases. Only
two were drawn in by the tips of the needles, and these were not
real exceptions, as one was drawn in for a very short distance, and
the two needles of the other cohered. Other pine-leaves were given
to worms kept in pots in a warm room, and here the result was
different; for out of 42 leaves drawn into the burrows, no less
than i6 were drawn in by the tips of the needles. These worms,
however, worked in a careless or slovenly manner; for the leaves
were often drawn in to only a small depth; sometimes they were
merely heaped over the mouths of the burrows, and sometimes none
were drawn in. I believe that this carelessness may be accounted
for either by the warmth of the air, or by its dampness, as the
pots were covered by glass plates; the worms consequently did not
care about plugging up their holes effectually. Pots tenanted by
worms and covered with a net which allowed the free entrance of
air, were left out of doors for several nights, and now 72 leaves
were all properly drawn in by their bases.
It might perhaps be inferred from the facts as yet given, that
worms somehow gain a general notion of the shape or structure of
pine-leaves, and perceive that it is necessary for them to seize
the base where the two needles are conjoined. But the following
cases make this more than doubtful. The tips of a large number of
needles of P. austriaca were cemented together with shell-lac
dissolved in alcohol, and were kept for some days, until, as I
believe, all odour or taste had been lost; and they were then
scattered on the ground where no pine-trees grew, near burrows from
which the plugging had been removed. Such leaves could have been
drawn into the burrows with equal ease by either end; and judging
from analogy and more especially from the case presently to be
given of the petioles of Clematis montana, I expected that the apex
would have been preferred. But the result was that out of 121
leaves with the tips cemented, which were drawn into burrows, 108
were drawn in by their bases, and only 13 by their tips. Thinking
that the worms might possibly perceive and dislike the smell or
taste of the shell-lac, though this was very improbable, especially
after the leaves had been left out during several nights, the tips
of the needles of many leaves were tied together with fine thread.
Of leaves thus treated 150 were drawn into burrows--123 by the base
and 27 by the tied tips; so that between four land five times as
many were drawn in by the base as by the tip. It is possible that
the short cut-off ends of the thread with which they were tied, may
have tempted the worms to drag in a larger proportional number by
the tips than when cement was used. Of the leaves with tied and
cemented tips taken together (271 in number) 85 per cent. were
drawn in by the base and 15 per cent. by the tips. We may
therefore infer that it is not the divergence of the two needles
which leads worms in a state of nature almost invariably to drag
pine-leaves into their burrows by the base. Nor can it be the
sharpness of the points of the needles which determines them; for,
as we have seen, many leaves with the points cut off were drawn in
by their bases. We are thus led to conclude, that with pine-leaves
there must be something attractive to worms in the base,
notwithstanding that few ordinary leaves are drawn in by the base
or foot-stalk.
Petioles.--We will now turn to the petioles or foot-stalks of
compound leaves, after the leaflets have fallen off. Those from
Clematis montana, which grew over a verandah, were dragged early in
January in large numbers into the burrows on an adjoining gravel-
walk, lawn, and flower-bed. These petioles vary from 2.5 to 4.5
inches in length, are rigid and of nearly uniform thickness, except
close to the base where they thicken rather abruptly, being here
about twice as thick as in any other part. The apex is somewhat
pointed, but soon withers and is then easily broken off. Of these
petioles, 314 were pulled out of burrows in the above specified
sites; and it was found that 76 per cent. had been drawn in by
their tips, and 24 per cent by their bases; so that those drawn in
by the tip were a little more than thrice as many as those drawn in
by the base. Some of those extracted from the well-beaten gravel-
walk were kept separate from the others; and of these (59 in
number) nearly five times as many had been drawn in by the tip as
by the base; whereas of those extracted from the lawn and flower-
bed, where from the soil yielding more easily, less care would be
necessary in plugging up the burrows, the proportion of those drawn
in by the tip (130) to those drawn in by the base (48) was rather
less than three to one. That these petioles had been dragged into
the burrows for plugging them up, and not for food, was manifest,
as neither end, as far as I could see, had been gnawed. As several
petioles are used to plug up the same burrow, in one case as many
as 10, and in another case as many as 15, the worms may perhaps at
first draw in a few by the thicker end so as to save labour; but
afterwards a large majority are drawn in by the pointed end, in
order to plug up the hole securely.
The fallen petioles of our native ash-tree were next observed, and
the rule with most objects, viz., that a large majority are dragged
into the burrows by the more pointed end, had not here been
followed; and this fact much surprised me at first. These petioles
vary in length from 5 to 8.5 inches; they are thick and fleshy
towards the base, whence they taper gently towards the apex, which
is a little enlarged and truncated where the terminal leaflet had
been originally attached. Under some ash-trees growing in a grass-
field, 229 petioles were pulled out of worm burrows early in
January, and of these 51.5 per cent. had been drawn in by the base,
and 48.5 per cent. by the apex. This anomaly was however readily
explained as soon as the thick basal part was examined; for in 78
out of 103 petioles, this part had been gnawed by worms, just above
the horse-shoe shaped articulation. In most cases there could be
no mistake about the gnawing; for ungnawed petioles which were
examined after being exposed to the weather for eight additional
weeks had not become more disintegrated or decayed near the base
than elsewhere. It is thus evident that the thick basal end of the
petiole is drawn in not solely for the sake of plugging up the
mouths of the burrows, but as food. Even the narrow truncated tips
of some few petioles had been gnawed; and this was the case in 6
out of 37 which were examined for this purpose. Worms, after
having drawn in and gnawed the basal end, often push the petioles
out of their burrows; and then drag in fresh ones, either by the
base for food, or by the apex for plugging up the mouth more
effectually. Thus, out of 37 petioles inserted by their tips, 5
had been previously drawn in by the base, for this part had been
gnawed. Again, I collected a handful of petioles lying loose on
the ground close to some plugged-up burrows, where the surface was
thickly strewed with other petioles which apparently had never been
touched by worms; and 14 out of 47 (i.e. nearly one-third), after
having had their bases gnawed had been pushed out of the burrows
and were now lying on the ground. From these several facts we may
conclude that worms draw in some petioles of the ash by the base to
serve as food, and others by the tip to plug up the mouths of their
burrows in the most efficient manner.
The petioles of Robinia pseudo-acacia vary from 4 or 5 to nearly 12
inches in length; they are thick close to the base before the
softer parts have rotted off, and taper much towards the upper end.
They are so flexible that I have seen some few doubled up and thus
drawn into the burrows of worms. Unfortunately these petioles were
not examined until February, by which time the softer parts had
completely rotted off, so that it was impossible to ascertain
whether worms had gnawed the bases, though this is in itself
probable. Out of 121 petioles extracted from burrows early in
February, 68 were imbedded by the base, and 53 by the apex. On
February 5 all the petioles which had been drawn into the burrows
beneath a Robinia, were pulled up; and after an interval of eleven
days, 35 petioles had been again dragged in, 19 by the base, and 16
by the apex. Taking these two lots together, 56 per cent. were
drawn in by the base, and 44 per cent. by the apex. As all the
softer parts had long ago rotted off, we may feel sure, especially
in the latter case, that none had been drawn in as food. At this
season, therefore, worms drag these petioles into their burrows
indifferently by either end, a slight preference being given to the
base. This latter fact may be accounted for by the difficulty of
plugging up a burrow with objects so extremely thin as are the
upper ends. In support of this view, it may be stated that out of
the 16 petioles which had been drawn in by their upper ends, the
more attenuated terminal portion of 7 had been previously broken
off by some accident.
Triangles of paper.--Elongated triangles were cut out of moderately
stiff writing-paper, which was rubbed with raw fat on both sides,
so as to prevent their becoming excessively limp when exposed at
night to rain and dew. The sides of all the triangles were three
inches in length, with the bases of 120 one inch, and of the other
183 half an inch in length. These latter triangles were very
narrow or much acuminated. {32} As a check on the observations
presently to be given, similar triangles in a damp state were
seized by a very narrow pair of pincers at different points and at
all inclinations with reference to the margins, and were then drawn
into a short tube of the diameter of a worm-burrow. If seized by
the apex, the triangle was drawn straight into the tube, with its
margins infolded; if seized at some little distance from the apex,
for instance at half an inch, this much was doubled back within the
tube. So it was with the base and basal angles, though in this
case the triangles offered, as might have been expected, much more
resistance to being drawn in. If seized near the middle the
triangle was doubled up, with the apex and base left sticking out
of the tube. As the sides of the triangles were three inches in
length, the result of their being drawn into a tube or into a
burrow in different ways, may be conveniently divided into three
groups: those drawn in by the apex or within an inch of it; those
drawn in by the base or within an inch of it; and those drawn in by
any point in the middle inch.
In order to see how the triangles would be seized by worms, some in
a damp state were given to worms kept in confinement. They were
seized in three different manners in the case of both the narrow
and broad triangles: viz., by the margin; by one of the three
angles, which was often completely engulfed in their mouths; and
lastly, by suction applied to any part of the flat surface. If
lines parallel to the base and an inch apart, are drawn across a
triangle with the sides three inches in length, it will be divided
into three parts of equal length. Now if worms seized
indifferently by chance any part, they would assuredly seize on the
basal part or division far oftener than on either of the two other
divisions. For the area of the basal to the apical part is as 5 to
1, so that the chance of the former being drawn into a burrow by
suction, will be as 5 to 1, compared with the apical part. The
base offers two angles and the apex only one, so that the former
would have twice as good a chance (independently of the size of the
angles) of being engulfed in a worm's mouth, as would the apex. It
should, however, be stated that the apical angle is not often
seized by worms; the margin at a little distance on either side
being preferred. I judge of this from having found in 40 out of 46
cases in which triangles had been drawn into burrows by their
apical ends, that the tip had been doubled back within the burrow
for a length of between 1/20 of an inch and 1 inch. Lastly, the
proportion between the margins of the basal and apical parts is as
3 to 2 for the broad, and 2.5 to 2 for the narrow triangles. From
these several considerations it might certainly have been expected,
supposing that worms seized hold of the triangles by chance, that a
considerably larger proportion would have been dragged into the
burrows by the basal than by the apical part; but we shall
immediately see how different was the result.
Triangles of the above specified sizes were scattered on the ground
in many places and on many successive nights near worm-burrows,
from which the leaves, petioles, twigs, &c., with which they had
been plugged, were removed. Altogether 303 triangles were drawn by
worms into their burrows: 12 others were drawn in by both ends,
but as it was impossible to judge by which end they had been first
seized, these are excluded. Of the 303, 62 per cent. had been
drawn in by the apex (using this term for all drawn in by the
apical part, one inch in length); 15 per cent. by the middle; and
23 per cent. by the basal part. If they had been drawn
indifferently by any point, the proportion for the apical, middle
and basal parts would have been 33.3 per cent. for each; but, as we
have just seen, it might have been expected that a much larger
proportion would have been drawn in by the basal than by any other
part. As the case stands, nearly three times as many were drawn in
by the apex as by the base. If we consider the broad triangles by
themselves, 59 per cent. were drawn in by the apex, 25 per cent. by
the middle, and 16 per cent. by the base. Of the narrow triangles,
65 per cent. were drawn in by the apex, 14 per cent, by the middle,
and 21 per cent. by the base; so that here those drawn in by the
apex were more than 3 times as many as those drawn in by the base.
We may therefore conclude that the manner in which the triangles
are drawn into the burrows is not a matter of chance.
In eight cases, two triangles had been drawn into the same burrow,
and in seven of these cases, one had been drawn in by the apex and
the other by the base. This again indicates that the result is not
determined by chance. Worms appear sometimes to revolve in the act
of drawing in the triangles, for five out of the whole lot had been
wound into an irregular spire round the inside of the burrow.
Worms kept in a warm room drew 63 triangles into their burrows;
but, as in the case of the pine-leaves, they worked in a rather
careless manner, for only 44 per cent. were drawn in by the apex,
22 per cent. by the middle, and 33 per cent. by the base. In five
cases, two triangles were drawn into the same burrow.
It may be suggested with much apparent probability that so large a
proportion of the triangles were drawn in by the apex, not from the
worms having selected this end as the most convenient for the
purpose, but from having first tried in other ways and failed.
This notion was countenanced by the manner in which worms in
confinement were seen to drag about and drop the triangles; but
then they were working carelessly. I did not at first perceive the
importance of this subject, but merely noticed that the bases of
those triangles which had been drawn in by the apex, were generally
clean and not crumpled. The subject was afterwards attended to
carefully. In the first place several triangles which had been
drawn in by the basal angles, or by the base, or a little above the
base, and which were thus much crumpled and dirtied, were left for
some hours in water and were then well shaken while immersed; but
neither the dirt nor the creases were thus removed. Only slight
creases could be obliterated, even by pulling the wet triangles
several times through my fingers. Owing to the slime from the
worms' bodies, the dirt was not easily washed off. We may
therefore conclude that if a triangle, before being dragged in by
the apex, had been dragged into a burrow by its base with even a
slight degree of force, the basal part would long retain its
creases and remain dirty. The condition of 89 triangles (65 narrow
and 24 broad ones), which had been drawn in by the apex, was
observed; and the bases of only 7 of them were at all creased,
being at the same time generally dirty. Of the 82 uncreased
triangles, 14 were dirty at the base; but it does not follow from
this fact that these had first been dragged towards the burrows by
their bases; for the worms sometimes covered large portions of the
triangles with slime, and these when dragged by the apex over the
ground would be dirtied; and during rainy weather, the triangles
were often dirtied over one whole side or over both sides. If the
worms had dragged the triangles to the mouths of their burrows by
their bases, as often as by their apices, and had then perceived,
without actually trying to draw them into the burrow, that the
broader end was not well adapted for this purpose--even in this
case a large proportion would probably have had their basal ends
dirtied. We may therefore infer--improbable as is the inference--
that worms are able by some means to judge which is the best end by
which to draw triangles of paper into their burrows.
The percentage results of the foregoing observations on the manner
in which worms draw various kinds of objects into the mouths of
their burrows may be abridged as follows:-
Drawn
into the Drawn in, Drawn in,
Nature of Object. burrows, by or by or
by or near near
near the the the
apex. middle. base.
Leaves of various kinds 80 11 9
- of the Lime, basal margin
of blade broad, apex
acuminated 79 17 4
- of a Laburnum, basal part of
blade as narrow as, or some-
times little narrower than
the apical part 63 10 27
- of the Rhododendron, basal
part of blade often narrower
than the apical part 34 ... 66
- of Pine-trees, consisting of
two needles arising from a
common base ... ... 100
Petioles of a Clematis,
somewhat pointed at the apex,
and blunt at the base 76 ... 24
- of the Ash, the thick basal
end often drawn in to serve
as food 48.5 ... 51.5
- of Robinia, extremely thin,
especially towards the apex,
so as to be ill-fitted for
plugging up the burrows 44 ... 56
Triangles of paper, of the
two sizes 62 15 23
- of the broad ones alone 59 25 16
- of the narrow ones alone 65 14 21
If we consider these several cases, we can hardly escape from the
conclusion that worms show some degree of intelligence in their
manner of plugging up their burrows. Each particular object is
seized in too uniform a manner, and from causes which we can
generally understand, for the result to be attributed to mere
chance. That every object has not been drawn in by its pointed
end, may be accounted for by labour having been saved through some
being inserted by their broader or thicker ends. No doubt worms
are led by instinct to plug up their burrows; and it might have
been expected that they would have been led by instinct how best to
act in each particular case, independently of intelligence. We see
how difficult it is to judge whether intelligence comes into play,
for even plants might sometimes be thought to be thus directed; for
instance when displaced leaves re-direct their upper surfaces
towards the light by extremely complicated movements and by the
shortest course. With animals, actions appearing due to
intelligence may be performed through inherited habit without any
intelligence, although aboriginally thus acquired. Or the habit
may have been acquired through the preservation and inheritance of
beneficial variations of some other habit; and in this case the new
habit will have been acquired independently of intelligence
throughout the whole course of its development. There is no a
priori improbability in worms having acquired special instincts
through either of these two latter means. Nevertheless it is
incredible that instincts should have been developed in reference
to objects, such as the leaves of petioles of foreign plants,
wholly unknown to the progenitors of the worms which act in the
described manner. Nor are their actions so unvarying or inevitable
as are most true instincts.
As worms are not guided by special instincts in each particular
case, though possessing a general instinct to plug up their
burrows, and as chance is excluded, the next most probable
conclusion seems to be that they try in many different ways to draw
in objects, and at last succeed in some one way. But it is
surprising that an animal so low in the scale as a worm should have
the capacity for acting in this manner, as many higher animals have
no such capacity. For instance, ants may be seen vainly trying to
drag an object transversely to their course, which could be easily
drawn longitudinally; though after a time they generally act in a
wiser manner, M. Fabre states {33} that a Sphex--an insect
belonging to the same highly-endowed order with ants--stocks its
nest with paralysed grass-hoppers, which are invariably dragged
into the burrow by their antennae. When these were cut off close
to the head, the Sphex seized the palpi; but when these were
likewise cut off, the attempt to drag its prey into the burrow was
given up in despair. The Sphex had not intelligence enough to
seize one of the six legs or the ovipositor of the grasshopper,
which, as M. Fabre remarks, would have served equally well. So
again, if the paralysed prey with an egg attached to it be taken
out of the cell, the Sphex after entering and finding the cell
empty, nevertheless closes it up in the usual elaborate manner.
Bees will try to escape and go on buzzing for hours on a window,
one half of which has been left open. Even a pike continued during
three months to dash and bruise itself against the glass sides of
an aquarium, in the vain attempt to seize minnows on the opposite
side. {34} A cobra-snake was seen by Mr. Layard {35} to act much
more wisely than either the pike or the Sphex; it had swallowed a
toad lying within a hole, and could not withdraw its head; the toad
was disgorged, and began to crawl away; it was again swallowed and
again disgorged; and now the snake had learnt by experience, for it
seized the toad by one of its legs and drew it out of the hole.
The instincts of even the higher animals are often followed in a
senseless or purposeless manner: the weaver-bird will
perseveringly wind threads through the bars of its cage, as if
building a nest: a squirrel will pat nuts on a wooden floor, as if
he had just buried them in the ground: a beaver will cut up logs
of wood and drag them about, though there is no water to dam up;
and so in many other cases.
Mr. Romanes, who has specially studied the minds of animals,
believes that we can safely infer intelligence, only when we see an
individual profiting by its own experience. By this test the cobra
showed some intelligence; but this would have been much plainer if
on a second occasion he had drawn a toad out of a hole by its leg.
The Sphex failed signally in this respect. Now if worms try to
drag objects into their burrows first in one way and then in
another, until they at last succeed, they profit, at least in each
particular instance, by experience.
But evidence has been advanced showing that worms do not habitually
try to draw objects into their burrows in many different ways.
Thus half-decayed lime-leaves from their flexibility could have
been drawn in by their middle or basal parts, and were thus drawn
into the burrows in considerable numbers; yet a large majority were
drawn in by or near the apex. The petioles of the Clematis could
certainly have been drawn in with equal ease by the base and apex;
yet three times and in certain cases five times as many were drawn
in by the apex as by the base. It might have been thought that the
foot-stalks of leaves would have tempted the worms as a convenient
handle; yet they are not largely used, except when the base of the
blade is narrower than the apex. A large number of the petioles of
the ash are drawn in by the base; but this part serves the worms as
food. In the case of pine-leaves worms plainly show that they at
least do not seize the leaf by chance; but their choice does not
appear to be determined by the divergence of the two needles, and
the consequent advantage or necessity of drawing them into their
burrows by the base. With respect to the triangles of paper, those
which had been drawn in by the apex rarely had their bases creased
or dirty; and this shows that the worms had not often first tried
to drag them in by this end.
If worms are able to judge, either before drawing or after having
drawn an object close to the mouths of their burrows, how best to
drag it in, they must acquire some notion of its general shape.
This they probably acquire by touching it in many places with the
anterior extremity of their bodies, which serves as a tactile
organ. It may be well to remember how perfect the sense of touch
becomes in a man when born blind and deaf, as are worms. If worms
have the power of acquiring some notion, however rude, of the shape
of an object and of their burrows, as seems to be the case, they
deserve to be called intelligent; for they then act in nearly the
same manner as would a man under similar circumstances.
To sum up, as chance does not determine the manner in which objects
are drawn into the burrows, and as the existence of specialized
instincts for each particular case cannot be admitted, the first
and most natural supposition is that worms try all methods until
they at last succeed; but many appearances are opposed to such a
supposition. One alternative alone is left, namely, that worms,
although standing low in the scale of organization, possess some
degree of intelligence. This will strike every one as very
improbable; but it may be doubted whether we know enough about the
nervous system of the lower animals to justify our natural distrust
of such a conclusion. With respect to the small size of the
cerebral ganglia, we should remember what a mass of inherited
knowledge, with some power of adapting means to an end, is crowded
into the minute brain of a worker-ant.
Means by which worms excavate their burrows.--This is effected in
two ways; by pushing away the earth on all sides, and by swallowing
it. In the former case, the worm inserts the stretched out and
attenuated anterior extremity of its body into any little crevice,
or hole; and then, as Perrier remarks, {36} the pharynx is pushed
forwards into this part, which consequently swells and pushes away
the earth on all sides. The anterior extremity thus serves as a
wedge. It also serves, as we have before seen, for prehension and
suction, and as a tactile organ. A worm was placed on loose mould,
and it buried itself in between two and three minutes. On another
occasion four worms disappeared in 15 minutes between the sides of
the pot and the earth, which had been moderately pressed down. On
a third occasion three large worms and a small one were placed on
loose mould well mixed with fine sand and firmly pressed down, and
they all disappeared, except the tail of one, in 35 minutes. On a
fourth occasion six large worms were placed on argillaceous mud
mixed with sand firmly pressed down, and they disappeared, except
the extreme tips of the tails of two of them, in 40 minutes. In
none of these cases, did the worms swallow, as far as could be
seen, any earth. They generally entered the ground close to the
sides of the pot.
A pot was next filled with very fine ferruginous sand, which was
pressed down, well watered, and thus rendered extremely compact. A
large worm left on the surface did not succeed in penetrating it
for some hours, and did not bury itself completely until 25 hrs. 40
min. had elapsed. This was effected by the sand being swallowed,
as was evident by the large quantity ejected from the vent, long
before the whole body had disappeared. Castings of a similar
nature continued to be ejected from the burrow during the whole of
the following day.
As doubts have been expressed by some writers whether worms ever
swallow earth solely for the sake of making their burrows, some
additional cases may be given. A mass of fine reddish sand, 23
inches in thickness, left on the ground for nearly two years, had
been penetrated in many places by worms; and their castings
consisted partly of the reddish sand and partly of black earth
brought up from beneath the mass. This sand had been dug up from a
considerable depth, and was of so poor a nature that weeds could
not grow on it. It is therefore highly improbable that it should
have been swallowed by the worms as food. Again in a field near my
house the castings frequently consist of almost pure chalk, which
lies at only a little depth beneath the surface; and here again it
is very improbable that the chalk should have been swallowed for
the sake of the very little organic matter which could have
percolated into it from the poor overlying pasture. Lastly, a
casting thrown up through the concrete and decayed mortar between
the tiles, with which the now ruined aisle of Beaulieu Abbey had
formerly been paved, was washed, so that the coarser matter alone
was left. This consisted of grains of quartz, micaceous slate,
other rocks, and bricks or tiles, many of them from 1/20 to 1/10
inch in diameter. No one will suppose that these grains were
swallowed as food, yet they formed more than half of the casting,
for they weighed 19 grains, the whole casting having weighed 33
grains. Whenever a worm burrows to a depth of some feet in
undisturbed compact ground, it must form its passage by swallowing
the earth; for it is incredible that the ground could yield on all
sides to the pressure of the pharynx when pushed forwards within
the worm's body.
That worms swallow a larger quantity of earth for the sake of
extracting any nutritious matter which it may contain than for
making their burrows, appears to me certain. But as this old
belief has been doubted by so high an authority as Claparede,
evidence in its favour must be given in some detail. There is no a
priori improbability in such a belief, for besides other annelids,
especially the Arenicola marina, which throws up such a profusion
of castings on our tidal sands, and which it is believed thus
subsists, there are animals belonging to the most distinct classes,
which do not burrow, but habitually swallow large quantities of
sand; namely, the molluscan Onchidium and many Echinoderms. {37}
If earth were swallowed only when worms deepened their burrows or
made new ones, castings would be thrown up only occasionally; but
in many places fresh castings may be seen every morning, and the
amount of earth ejected from the same burrow on successive days is
large. Yet worms do not burrow to a great depth, except when the
weather is very dry or intensely cold. On my lawn the black
vegetable mould or humus is only about 5 inches in thickness, and
overlies light-coloured or reddish clayey soil: now when castings
are thrown up in the greatest profusion, only a small proportion
are light coloured, and it is incredible that the worms should
daily make fresh burrows in every direction in the thin superficial
layer of dark-coloured mould, unless they obtained nutriment of
some kind from it. I have observed a strictly analogous case in a
field near my house where bright red clay lay close beneath the
surface. Again on one part of the Downs near Winchester the
vegetable mould overlying the chalk was found to be only from 3 to
4 inches in thickness; and the many castings here ejected were as
black as ink and did not effervesce with acids; so that the worms
must have confined themselves to this thin superficial layer of
mould, of which large quantities were daily swallowed. In another
place at no great distance the castings were white; and why the
worms should have burrowed into the chalk in some places and not in
others, I am unable to conjecture.
Two great piles of leaves had been left to decay in my grounds, and
months after their removal, the bare surface, several yards in
diameter, was so thickly covered during several months with
castings that they formed an almost continuous layer; and the large
number of worms which lived here must have subsisted during these
months on nutritious matter contained in the black earth.
The lowest layer from another pile of decayed leaves mixed with
some earth was examined under a high power, and the number of
spores of various shapes and sizes which it contained was
astonishingly great; and these crushed in the gizzards of worms may
largely aid in supporting them. Whenever castings are thrown up in
the greatest number, few or no leaves are drawn into the burrows;
for instance the turf along a hedgerow, about 200 yards in length,
was daily observed in the autumn during several weeks, and every
morning many fresh castings were seen; but not a single leaf was
drawn into these burrows. These castings from their blackness and
from the nature of the subsoil could not have been brought up from
a greater depth than 6 or 8 inches. On what could these worms have
subsisted during this whole time, if not on matter contained in the
black earth? On the other hand, whenever a large number of leaves
are drawn into the burrows, the worms seem to subsist chiefly on
them, for few earth-castings are then ejected on the surface. This
difference in the behaviour of worms at different times, perhaps
explains a statement by Claparede, namely, that triturated leaves
and earth are always found in distinct parts of their intestines.
Worms sometimes abound in places where they can rarely or never
obtain dead or living leaves; for instance, beneath the pavement in
well-swept courtyards, into which leaves are only occasionally
blown. My son Horace examined a house, one corner of which had
subsided; and he found here in the cellar, which was extremely
damp, many small worm-castings thrown up between the stones with
which the cellar was paved; and in this case it is improbable that
the worms could ever have obtained leaves. Mr. A. C. Horner
confirms this account, as he has seen castings in the cellars of
his house, which is an old one at Tonbridge.
But the best evidence, known to me, of worms subsisting for at
least considerable periods of time solely on the organic matter
contained in earth, is afforded by some facts communicated to me by
Dr. King. Near Nice large castings abound in extraordinary
numbers, so that 5 or 6 were often found within the space of a
square foot. They consist of fine, pale-coloured earth, containing
calcareous matter, which after having passed through the bodies of
worms and being dried, coheres with considerable force. I have
reason to believe that these castings had been formed by species of
Perichaeta, which have been naturalized here from the East. {38}
They rise like towers, with their summits often a little broader
than their bases, sometimes to a height of above 3 and often to a
height of 2.5 inches. The tallest of those which were measured was
3.3 inches in height and 1 inch in diameter. A small cylindrical
passage runs up the centre of each tower, through which the worm
ascends to eject the earth which it has swallowed, and thus to add
to its height. A structure of this kind would not allow leaves
being easily dragged from the surrounding ground into the burrows;
and Dr. King, who looked carefully, never saw even a fragment of a
leaf thus drawn in. Nor could any trace be discovered of the worms
having crawled down the exterior surfaces of the towers in search
of leaves; and had they done so, tracks would almost certainly have
been left on the upper part whilst it remained soft. It does not,
however, follow that these worms do not draw leaves into their
burrows during some other season of the year, at which time they
would not build up their towers.
From the several foregoing cases, it can hardly be doubted that
worms swallow earth, not only for the sake of making their burrows,
but for obtaining food. Hensen, however, concludes from his
analyses of mould that worms probably could not live on ordinary
vegetable mould, though he admits that they might be nourished to
some extent by leaf-mould. {39} But we have seen that worms
eagerly devour raw meat, fat, and dead worms; and ordinary mould
can hardly fail to contain many ova, larvae, and small living or
dead creatures, spores of cryptogamic plants, and micrococci, such
as those which give rise to saltpetre. These various organisms,
together with some cellulose from any leaves and roots not utterly
decayed, might well account for such large quantities of mould
being swallowed by worms. It may be worth while here to recall the
fact that certain species of Utricularia, which grow in damp places
in the tropics, possess bladders beautifully constructed for
catching minute subterranean animals; and these traps would not
have been developed unless many small animals inhabited such soil.
The depth to which worms penetrate, and the construction of their
burrows.--Although worms usually live near the surface, yet they
burrow to a considerable depth during long-continued dry weather
and severe cold. In Scandinavia, according to Eisen, and in
Scotland, according to Mr. Lindsay Car
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